1. Field of the Invention
The present invention relates to devices for the production of composite panels under the application of heat and pressure, and more particularly to heated panel presses for the production of chipboard panels, fiber panels, and the like, panel presses of this type being either single-layer or multi-layer presses and having a vertically movable press spar arranged above and opposite to a stationary press spar, both carrying heated pressure plates.
2. Description of the Prior Art
Large heated panel presses of the above-mentioned type are notorious for their problems encountered with respect to the maintenance of the required plane-parallelism of the heated pressure plates over a large number of pressing operations, as the pressure plates are alternatingly heated and cooled. The problems of heat distortion of the pressure plates are related to the fact that the latter are rather large in surface and have to be heated to an elevated temperature, so that a certain degree of uneven heat distribution within the structure of the supporting press spar is unavoidable. The latter leads to thermal stress and corresponding distortions of the pressure plate, with the result that the originally adjusted geometrically precise flatness of the pressure plates is soon lost.
Various approaches have been suggested in the past for a correction of this problem, one such proposal being that not only the pressure plates but also the supporting structure of the press spars should be heated, rather than only the pressure plates. Another approach suggests that the press spars should be cooled and that heat transfer barriers should be installed between the pressure plates and the supporting press spars, in order to prevent all heat transmission between them.
Known prior art panel presses which have such a heat transfer barrier are equipped with either a cooling grid and a special cooling plate, or an insulating layer against which the pressure plates are supported. This structure is subject to the shortcomings that a considerable number of machined surfaces are necessary, that corrosion is frequently encountered on these surfaces, and that the combined effects of cumulative machining tolerances, corrosion, and comparatively rapid local wear of the insulating layers amount to the end result that, even with a pressure plate machined to perfect flatness, no satisfactory pressing performance under maintenance of the necessary plane-parallelism over a substantial length of time is obtainable. The basic construction of such a press is disclosed, for example, in U.S. Pat. No. 3,594,867. Upper and lower press spars with insulated pressure plates are disclosed in U.S. Pat. Nos. 3,685,932 and 3,775,033.
One of the reasons for the above-mentioned problems is the tendency of the insulating layers to absorb humidity, thereby alternatingly swelling and contracting. The simultaneous expansion and contraction movements of the structural parts under the constant changes in temperature result in frictional displacements between the pressure plates and the insulating layers, so that the latter are subjected to rapid abrasion and premature destruction. Lastly, the pressure resistance and compressibility of the insulating layers was found to be changing as a function of the amount of humidity which has been absorbed by them.
The lack of dimensional stability of the insulating meterials and their inadequate long-term resistance to elevated temperatures and compression result in uneven wear and dimensional distortions of the insulating plates, so that the working surfaces of the pressure plates eventually become similarly distorted. The resultant work product is a composite pressed panel of uneven thickness, necessitating either reworking of the product, or complete rejection.
Other prior art solutions using the approach of compensatory heating or cooling of the entire press spar structure have the shortcoming that such a structure becomes very complex and costly, necessitating complicated temperature control devices. A further shortcoming of this approach relates to the fact that a considerable amount of time is necessary for the initial heating of such a structure, until a state is reached in which all the component parts of the press spar have an even temperature.
In my co-pending application Ser. No. 486,375, filed July 8, 1974, is disclosed a solution to the above problem, the invention suggesting the arrangement of a thermal barrier between each pressure plate and a number of transversely extending, longitudinally spaced uprights of the press spar structure, the thermal barrier being constituted essentially of a row of insulating pressure blocks of high pressure resistance, low heat conductivity, and little or no humidity absorption. These insulating pressure blocks are confined between upper and lower metallic shrouds presenting wear-resistant outer displacement surfaces with an anti-friction layer to the pressure plate and the press spar uprights in the opposite direction.
In another embodiment of the above-mentioned prior invention is suggested the arrangement of a bottom plate between the press spar uprights and the thermal barrier which is welded to the uprights. This bottom plate has substantially the same horizontal extent as the pressure plates. Rows of shrouded insulating blocks, arranged in alignment with the uprights of the spar structure, and intermediate loose insulating pads constitute the heat barrier. The bottom plate may further have arranged in it a system of heat transfer channels, as part of a press spar heating and/or cooling system.
It has now been found that under extreme temperature and pressure conditions encountered over an extended period of time, the insulating pressure blocks of the prior invention may fail in their pressure resistance, with the result that the plane-parallelism of the pressure plates is lost. A product of diminished quality, due to uneven thickness, is the result.